(a) The banding problem—White or light striations along the scan axis, often called “banding”, have been a commonly noted problem since the earliest days of incremental printing by inkjet and like printing devices, and ironically have yet to be fully resolved. A primary reason for the seeming stubbornness of these artifacts is that several different kinds and causes of banding occur, so that no single insight or cure can be possible.
Many banding types are now under much better control than originally. At the same time, however, new solutions for various banding problems continue to be needed because of rising quality standards and increasingly difficult operating conditions—due e.g. to marketplace demands for high throughput.
The present invention is directed mainly to banding that is caused by dot-placement error (DPE)—in other words, errors produced by printing elements (inkjet nozzles etc.) that are not aimed correctly. It also helps conceal banding due to elements that are not working at all or are weak, as for example inkjet nozzles that are plugged or whose firing heaters are not of the correct resistance.
In all such cases, rows of the image pixel grid that should be printed at some nominal saturation level are instead printed either lightly or not at all, and this is perceived as a white or light line across the image. Such lines may appear singly or in clusters—since the print-element failures that produce them often occur in groups.
(b) The search for a robust solution—In this field the term “robust” means resistant to varied severity of the problem and to a variety of operating conditions, including some conditions that may be unexpected. The act of aiming one single dot into a regular grid is actually the least robust method, as far as banding is concerned.
Such operation does display a good degree of peak image quality. The chance of achieving this optimum performance, however, is small due to the loose tolerances of economical printing systems (i.e. printing elements and positioning mechanisms).
A classical approach to mitigation of banding is multipass printing—and this technique is generally accepted since multipass printing also helps resolve several other important problems, though it does degrade throughput. Such printing divides each swath of an image into contributions made during several scans of the print-element array (e.g. pen) across the image.
This type of printing also advances the print medium, between scans or groups of scans, by some fraction of the array length. Each advance brings a different printing element (e.g. nozzle) of the array into alignment with a particular pixel row on the medium.
The idea is that any artifact due to a particular print element in an array—at some given row on a printing medium—tends to be buried and thus concealed in the contributions made by other elements of the same array. Classically, however, unfortunately each other element has its own job to do, so that actually light or white streaks due to impairment of one printing element remain discernible even though superposed upon patterns of colorant dots contributed by other elements.
In the past it has been supposed that multipass printing itself would suffice to adequately conceal banding of the light-line or white-line type. The error in such suppositions is due in part to escalating marketplace standards of quality—but also arises in large part from the evolving character of print-element malfunction.
Early incremental-printing systems, and particularly the printing-element arrays (nozzle plates etc.) were essentially machine tooled, and short, and rather precisely constructed—particularly as to aiming. Unfortunately these were both slow in printing (because of their shortness) and expensive. Progressively longer inkjet printing arrays have been made economically possible by the advent of tape-automated bonding (“TAB”) techniques with laser perforation, but only at the cost of considerable imprecision in dot placement (sometimes characterized as “directionality”).
Performance of TAB-fabricated printheads has now improved markedly. Nevertheless the capability of printing-element arrays to misdirect colorant continues to outstrip the capability of multipass printing to hide such phenomena. This is an example of the importance of robustness in solutions to the banding problem.
On the other hand, users may no longer be willing to wait quite as contentedly while a printer produces excellent image quality through painstaking six- or ten-pass printmodes. This is an example of the influence of escalating marketplace standards.
(c) Printmode techniques—Modern multipass printing has evolved greatly, with highly sophisticated strategies (“printmasking”) to allocate colorant deposition as among passes. These strategies include pseudorandom allocation, intended mainly to reduce patterning artifacts that come from repetitive interaction of cyclical mechanism errors with dither masks (rendition threshold matrices)—or even with repeating phenomena inherent in the more-elementary allocation strategies themselves. Sometimes these patterning artifacts, too, are called “banding” but for purposes of the present document that nomenclature is only confusing. What is of interest here is light- or white-line banding.
These strategies do incidentally help to swamp out the effects of any individual element malfunction in a complex of patterns, both intended and otherwise, generated by other elements. These methodologies are meritorious and serve their purpose well; overall, however, the correction of white- and light-line banding is not the main function of printmasking; and such banding persists.
Another very modern printmasking development is the use of print-element usage weighting, and complementary replacement regimens, in attempts to eliminate image-quality degradation due to known malfunctioning elements. The previously mentioned document of Garcia-Reyero, and other references cited in that document, introduce a great body of such technique.
All such efforts, however, require some sort of testing to identify malfunctioning elements; one such approach is presented in the above-mentioned document of Cluet. Techniques that require testing are somewhat disfavored in that they implicate operational delays and additional expensive apparatus—as well as costs for colorant (and sometimes printing medium) consumed in the tests.
Hence it remains desirable to find ways to eliminate or at least greatly reduce the appearance of light- or white-line banding without testing the printing elements. A robust solution must be one that deals effectively with dot-placement error that occurs in a variety of forms and intensities.
(d) Etiology of banding—The foregoing discussions point to an important need for deeper understanding of the detailed causes of white/light-line banding—and in particular its sensitivity to both the character and severity of dot-placement error. The present inventor has considered and experimented in this area very extensively, and along the way to the present invention has developed important insights into these facets of the banding problem.
These insights thus are no part of the related art but rather are regarded as part of the creative processes underlying the present invention. Accordingly they will be reserved for the following sections of this document that relate to the invention.
(e) Use of two or more drops—For the present section it may be noted how the prior art has applied more than one quantum of colorant (e.g. inkdrop) to a printing medium. These observations will be presented in a conceptual framework that will be useful for later discussions.
One relatively primitive technique is to use two quanta wherever any colorant is to be applied. In other words—on a scale from zero 121 (FIG. 2) to full saturation 122 (for instance solid black)—when the first step is taken away from zero inking 121 in highlight regions, it is taken by printing not one but two inkdrops in some pixel.
At the other end 122 of the scale when the last step is taken to achieve fullest available saturation, it is taken by adding not just one more last drop in some pixel but rather two. The continuum between these extreme points is followed in exactly the same way, defining a linear gradation 123 (FIG. 2) with two inkdrops (or other fixed count of quanta, depending on the writing system) added at each increment. When those last two drops have been added, then either zero or two dots have been placed into each pixel in the grid—or, for a system that does not use every pixel, into each pixel that is to be addressed at all.
(This full-usage pixel-count condition represents “full coverage” or “100% coverage” for the particular writing system. For purposes of this document, the working definition of “full coverage” or “100% coverage” is thus not a matter of calculating inked area. Rather it is a matter of counting colorant quanta, e.g. dots, per pixel—for comparison with the total number of quanta per pixel employed or permitted.)
The objectives of such double-dot-always operation may include obtaining, at each point along the scale, better calorimetric saturation than available with singledrop increments—and may also include providing finer drops and thereby better liquid control. This technique is sometimes called “dumb double dotting”.
As to banding, for reasons that will later become clear, dumb double dotting is slightly more robust than single-drop printing. Accompanying granularity, however, is very high. Moreover this type of printing is not compatible with some printing media—and fails to completely resolve the banding problem.
Conventional orderly multilevel printing proceeds by a different sequence. In addition to the highlight end 121 (FIG. 3) and shadow end 122 of the scale, here there is an additional, intermediate breakpoint 124 which represents the greatest saturation attainable with single dotting. Because the later and higher-dot-count condition 122 exists, however, breakpoint 124 is not “full coverage” or “100% coverage”.
In developing progressive fractions of area fill, this system first assigns one individual drop for placement in each one of a progressively rising number of pixels—following a linear relation 125 (FIG. 3) to the breakpoint 124. This linear curve represents the number of pixels holding one dot.
At the breakpoint 124 one dot is placed in each pixel—or, again, in each pixel that is to be addressed at all. As suggested above, although this pixel count may yield maximum single-dot saturation or perhaps even “full single-dot coverage”, for purposes of this document it will not be identified as “full coverage” or “100% coverage”.
Thereafter for continuing higher fractions of area fill it is necessary to begin to add another dot to some pixels. The number 126 of pixels holding one dot thus declines while the difference is taken up by a complementary linear curve 127, representing the number of pixels holding two dots.
The latter line ascends to the above-identified fullsaturation, “full coverage” or “100% coverage” point 122. In this region between the breakpoint 124 and the shadow or full-saturation end 122 of the scale, the total number of inked pixels is a sum of single-dot pixels 126 and double-dot pixels 127.
The possibility remains, however, that some pixels are entirely unused. As will shortly be seen, for instance, some systems use only every other pixel; such pixel structure has been used particularly in conjunction with oversize dots, relative to the grid pitch.
Still within the prior art, conventional orderly multilevel printing is capable of managing still larger numbers of dots per pixel. This is accomplished by extension of the same regimen just described.
More specifically, the system first adds not one breakpoint 124—having the same significance described above—but also a second intermediate breakpoint 131 (FIG. 4) related to the greatest saturation available with double dotting; the system then connects the several critical states linearly as before. Thus the first ascending segment 125 is still the growing number of pixels holding one dot; and the second, falling segment is the decline of such pixels as the number 127 of pixels holding two dots rises.
When that latter curve 127, however, now reaches full coverage by two dots per pixel—or per pixel that is to receive any dots at all—the saturation has now only reached the second breakpoint 131. Here, analogously, the number of two-dot pixels declines 132 (FIG. 4) while a new regime 133 namely the number of three-dot pixels completes the rise to full three-dot coverage 122.
In this region the total number of pixels is the sum of double-dot pixels 132 and triple-dot pixels 133. At the 100%-coverage point 122, three dots reside in every pixel—or, here again, at least every pixel that is ever to receive any dots.
The multilevel “orderly” methods have potential for improvement relative to binary printing, because a single drop can be smaller than before. Therefore these methodologies can improve granularity relative to binary printing—but no improvement is obtained in the initial banding behavior, in the region from zero area fill 121 to the point 124 where each pixel is occupied by one drop. Unfortunately it is in the upper ends of these ranges, i. e. the middle tones, where banding is most conspicuous to the eye.
There is another noteworthy variant of a “checkerboard” system—i. e., a rectangular pixel grid in which the only pixels used are those in a checkerboard pattern. Those of ordinary skill in this field will appreciate that such a system in principle can be made to use, say, the alternate checkerboard positions for the third dot set 133 or the second dot set 127, or both.
In such a case, dots in the third or second set are centered between, not on, dots in the first set 125. It will also be clear that for patent purposes this is merely an equivalent of a basic checkerboard system in which all three sets share the same pixels.
(f) Conclusion—Obstinate problems of white-line and light-line banding thus continue to impede achievement of uniformly excellent inkjet printing—at high throughput—on all industrially important printing media. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.